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Spectroscopic Studies of Physical and Electronic Structure in Transition Metal Oxide Photocatalysts

  • Author(s): Choing, Stephanie N Choing
  • Advisor(s): Cuk, Tanja
  • et al.
Abstract

The complex relationship between electronic structure and physical structure in both molecular and solid state photocatalytic systems is examined. Ultrafast transient optical absorption spectroscopy and x-ray photoelectron spectroscopy are the main techniques used to investigate the underlying properties that affect the reactivity of various photocatalysts. The two respective spectroscopies are exceptionally suited to observing the ultrafast kinetics and interfacial behavior of these systems. In this dissertation, two systems are investigated-- one a vanadium-based organometallic complex and the other strontium titanate, a wide band gap metal oxide semiconductor. The relaxation dynamics of a chelated oxovanadium (V) ligand-to-metal charge transfer complex are measured by ultrafast transient absorption. A combination of theoretical and experimental work indicate that the observed excited state lifetime in this molecular complex, which is an order of magnitude longer than those of metal-to-ligand charge transfer iron compounds, is derived uniquely from geometric constraints on its radiative and nonradiative relaxation pathways. Transient optical reflectance measurements are also carried out on niobium-doped strontium titanate at the solid-liquid interface to identify midgap radical states that could be implicated in the water oxidation mechanism. The creation of localized radical hole species is determined to be a surface-site limited process, which is highly dependent on the in situ reaction conditions. The application of voltage is shown to induce a significant redistribution of holes to the surface from valence band hole species, highlighting the capacity of the surface to accommodate localized charge under different conditions. Additional x-ray photoelectron spectroscopic measurements are taken under ambient pressure conditions to investigate water adsorption on the strontium titanate (001) surface. The effects of different variables-- surface termination, dopants, and photoelectrochemical surface treatment--on the surface wetting behavior are considered. Further quantitative analysis of the x-ray photoemission data is currently underway; details of the data processing and analysis procedures as well as a description of how these sample variables might manifest in our x-ray spectra are discussed.

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